Use reference to initialize class members

I always have the impression that I shouldn't use reference to initialize class member because

1. I don't know the lifetime of the reference and
2. If the reference value changes outside the class, the corresponding class member would also change to the new value.

But after testing the following code, I am confused...

class Test { public: Test(int& i) :m_i(i) {} int m_i; }; class Test3 { public: Test3(int& i) :m_i(i) {} const int& m_i; }; int main() { { std::cout << "\n// Test 1" << std::endl; int i = 10; Test oTest(i); printf("oTest.i = %d\n", oTest.m_i); i = 20; printf("oTest.i = %d\n", oTest.m_i); } { std::cout << "\n// Test 1.1" << std::endl; int* i = new int; *i = 10; Test oTest(*i); printf("oTest.i = %d\n", oTest.m_i); *i = 20; printf("oTest.i = %d\n", oTest.m_i); delete i; printf("oTest.i = %d\n", oTest.m_i); } { std::cout << "\n// Test 3" << std::endl; int i = 10; Test3 oTest(i); printf("oTest.i = %d\n", oTest.m_i); i = 20; printf("oTest.i = %d\n", oTest.m_i); } { std::cout << "\n// Test 3.1" << std::endl; int* i = new int; *i = 10; Test3 oTest(*i); printf("oTest.i = %d\n", oTest.m_i); *i = 20; printf("oTest.i = %d\n", oTest.m_i); delete i; printf("oTest.i = %d\n", oTest.m_i); } return 0; } 

Output is as follows:

// Test 1 oTest.i = 10 oTest.i = 10 <---- Why not 20? // Test 1.1 oTest.i = 10 oTest.i = 10 <---- Why not 20? oTest.i = 10 <---- Why not some garbage number? // Test 3 oTest.i = 10 oTest.i = 20 // Test 3.1 oTest.i = 10 oTest.i = 20 oTest.i = 20 <---- Why not some garbage number? 

Thanks very much for your comment.